FM modulator output control during turn on

Abstract

The present invention provides a system and a method to prevent undesired sounds during FM modulator turn on. The system includes an RF attenuator circuit, a delay switch circuit, an RF ramp up circuit, and audio ramp up circuit. The RF attenuator circuit significantly reduces the RF signal from the FM modulator when the FM modulator is initially turned on, which significantly reduce whistle caused by the RF signal from the modulator beating with the local oscillator in the radio receiver. After allowing time for the PLL to lock onto the RF signal, the time delay switch circuit becomes active and charges the RF ramp up circuit. The RF ramp up circuit gradually increases the RF signal by gradually deactivating the RF attenuator circuit. The gradual increase of the RF signal is slower than the response time of the radio AGC, thereby preventing any pop as the RF signal is provided to the radio receiver.

We claim: 
     
       1. A system adapted to eliminate undesired sounds during turn on of a FM modulator, the FM modulator having a RF output signal in communication with a radio receiver, a VCO modulation input, and a composite output signal, the system comprising:
 a RF attenuator circuit in communication with the RF output signal of the FM modulator and adapted to initially reduce the RF output signal; 
 a RF ramp up circuit in communication with the RF attenuator circuit and adapted to deactivate the RF attenuator circuit; 
 a delay switch circuit in communication with the RF ramp up circuit and adapted to delay activation of the RF ramp up circuit; and 
 an audio ramp up circuit in communication with the RF ramp-up circuit and the composite output Signal at the FM modulator, the audio ramp up circuit increasing the composite output signal. 
 
     
     
       2. The system according to  claim 1 , the audio ramp up circuit including a resistor, a capacitor, and a transistor. 
     
     
       3. The system according to  claim 1 , the time delay switch circuit including a first and second transistor, a resistor, and a capacitor. 
     
     
       4. The system according to  claim 3 , wherein the second transistor is a NPN transistor. 
     
     
       5. The system according to  claim 3 , the NPN transistor having a base in communication with the second transistor, an emitter in communication with an electrical ground, and a collector in communication with the RF ramp-up circuit. 
     
     
       6. The system according to  claim 3 , wherein the second transistor is a field effect transistor, the gate of the field effect transistor being in communication with the resistor and the capacitor, the source of the field effect transistor being in communication with an electrical ground, and the drain of the field effect transistor being in communication with the first transistor. 
     
     
       7. A system adapted to eliminate undesired sounds during turn on of a FM modulator, the FM modulator having a RF output signal in communication with a radio receiver, a VCO modulation input, and a composite output signal, the system comprising:
 a RF attenuator circuit in communication with the RF output signal of the FM modulator and adapted to initially reduce the RF output signal; 
 a RF ramp up circuit in communication with the RF attenuator circuit and adapted to deactivate the RF attenuator circuit; and 
 a delay switch circuit in communication with the RF ramp up circuit and adapted to delay activation of the RF ramp up circuit; and 
 wherein a first side of the resistor is in communication with the delay switch circuit, a second side of the resistor is in communication with the capacitor and the transistor, the capacitor being connected between the resistor and an electrical ground. 
 
     
     
       8. The system according to  claim 7 , the transistor of the audio ramp up circuit including a gate, a source, and a drain, wherein the gate is in communication with the resistor and the capacitor, the source is in connection with the composite output signal of the FM modulator, and the drain is in connection with the VCO modulation input. 
     
     
       9. A system adapted to eliminate undesired sounds during turn on of a FM modulator, the FM modulator having a RF output signal in communication with a radio receiver, a VCO modulation input, and a composite output signal, the system comprising:
 a RF attenuator circuit in communication with the RF output signal of the FM modulator and adapted to initially reduce the RF output signal; 
 a RF ramp up circuit in communication with the RF attenuator circuit and adapted to deactivate the RF attenuator circuit; and 
 a delay switch circuit in communication with the RF ramp up circuit and adapted to delay activation of the RF ramp up circuit, the RF attenuator circuit including a transistor, the transistor having a base, a collector, and an emitter; and 
 wherein the emitter is connected to a power source, the base is in communication with the RF ramp up circuit, and a collector is in communication with the RF signal of the FM modulator. 
 
     
     
       10. The system according to  claim 9 , wherein the transistor is a PNP transistor. 
     
     
       11. The system according to  claim 9 , further comprising an audio ramp up circuit in communication with the delay switch circuit and the composite output of the FM modulator. 
     
     
       12. The system according to  claim 11 , the audio ramp up circuit including a resistor, a capacitor, and a second transistor. 
     
     
       13. The system according to  claim 12 , wherein a first side of the resistor is in communication with the delay switch circuit, a second side of the resistor is in communication with the capacitor and the second transistor, the capacitor being connected between the resistor and an electrical ground. 
     
     
       14. The system according to  claim 13 , the second transistor of the audio ramp up circuit including a gate, a source, and a drain, wherein the gate is in communication with the resistor and the capacitor, the source is in connection with the composite output signal of the FM modulator, and the drain is in connection with the VCO modulation input of the FM modulator. 
     
     
       15. The system according to  claim 9 , the RF ramp up circuit including a capacitor and a first resistor. 
     
     
       16. The system according to  claim 15 , wherein a first side of the capacitor is in communication with an electrical ground and a second side of the capacitor is in communication with the first resistor and the delay switch circuit, the first resistor being connected between a voltage source and the RF attenuator circuit. 
     
     
       17. The system according to  claim 16 , the RF ramp-up circuit including a second resistor, the second resistor being connected between the RF attenuator and the delay switch circuit. 
     
     
       18. The system according to  claim 9 , the time delay switch circuit including a second and third transistor, a resistor, and a capacitor. 
     
     
       19. The system according to  claim 18 , wherein the second transistor is a NPN transistor, the NPN transistor having a base in communication with the third transistor, a emitter in communication with an electrical ground, and a collector in communication with the RF ramp-up circuit. 
     
     
       20. The system according to  claim 18 , wherein the third transistor is a field effect transistor, the gate of the field effect transistor being in communication with the resistor and the capacitor, the source of the field effect transistor being in communication with an electrical ground, and the drain of the field effect transistor being in communication with the second transistor. 
     
     
       21. A system adapted to eliminate undesired sounds during turn on of a FM modulator, the FM modulator having a RF output signal in communication with a radio receiver, a VCO modulation input, and a composite output signal, the system comprising:
 a RF attenuator circuit in communication with the RF output signal of the FM modulator and adapted to initially reduce the RF output signal; 
 a RF ramp up circuit in communication with the RF attenuator circuit and adapted to deactivate the RF attenuator circuit, the RF ramp up circuit including a capacitor and a first resistor; 
 a delay switch circuit in communication with the RF ramp up circuit and adapted to delay activation of the RF ramp up circuit; and 
 wherein a first side of the capacitor is in communication with an electrical ground and a second side of the capacitor is in communication with the first resistor and the delay switch circuit, the first resistor being connected between a voltage source and the RF attenuator circuit.